1. Field of the Invention
This invention relates to a process for producing a crystal article.
The present invention can be suitably used for, for example, SOI technique.
2. Related Background Art
In the prior art, the single crystalline thin film to be used for semiconductor electronic device, photoelectric device, etc. has been formed by epitaxial growth on a single crystalline substrate. However, for epitaxial growth of a single crystalline thin film on a single crystalline substrate, it is necessary to consider matching in lattice constant and coefficient of thermal expansion between the single crystalline material of the substrate and the epitaxial growth layer, and there has been involved the problem that the kinds of the substrate material were limited to extremely narrow scope for formation of a single crystalline layer capable of preparing a device of good quality.
On the other hand, in recent years, research and development have been actively done in three-dimensional integrated circuits for accomplishing high integration and multi-functionality by forming semiconductors by lamination in the direction normal to the substrate, as well as research and development for a large area semiconductor device such as a solar battery in which elements are arranged in an array on an inexpensive glass or switching transistor of liquid crystal picture elements, etc. and are becoming more active year by year.
What is common in these researches and developments, is that the technique to form a semiconductor film on an amorphous insulating material and form an electronic element such as transistor, etc. thereon is required. Among them, it has been particularly desired to have a technique to form a single crystalline semiconductor of high quality on an amorphous insulating material.
However, generally speaking, when a thin film is formed on an amorphous insulating substrate such as SiO.sub.2, etc., due to deficiency of long length order of the substrate material, the crystal structure of the deposited film becomes amorphous or polycrystalline, whereby it was very difficult to form a single crystalline semiconductor of high quality. Here, amorphous film refers to one with the state in which short length order to the minimum extent on the order of atom may be maintained, but there is no longer length order, while polycrystalline film refers to one in which single crystal grains having no specific crystal direction are gathered as separated with grain boundaries.
As a solution to the problems as mentioned above in the prior art, European Patent Application EP 244081 A1 proposes a method for forming a crystal by permitting a single crystal to grow on the single nucleus as the growing center, on the nucleation surface (S.sub.NDL) provided on a substrate surface which is constituted of a material sufficiently larger in nucleation density (ND) than the material constituting said substrate surface and has an area sufficiently small to the exent that only a single crystal can be grown, and by use of this method, it is possible to form a single crystal also on an insulating amorphous substrate.
In the invention shown in the above European Patent Application, when a single crystalline layer is formed on a substrate, a facet inherent in single crystal is generated, and therefore flattening is required for formation of a circuit element on the single crystalline layer. Particularly, when a concavity is formed on the substrate and a single crystal is formed in said concavity, it is necessary to remove the single crystal with good precision in conformity with the upper surface of the substrate in order to improve precision of fine working.